Epileptic seizures are characterized by excessive or abnormally synchronous neuronal activity. Neurologists recognize a wide variety of seizures. Partial onset seizures begin in one part of the brain; general onset seizures arise throughout the entire brain simultaneously. When partial onset seizures progress to involve much of the brain, they are said to have “secondarily generalized.” Some seizures result in the loss of conscious awareness and are termed “complex” seizures. So-called “simple” seizures may involve other symptoms, but consciousness is unimpaired. Seizure symptoms may include sensory distortions, involuntary movements, or loss of muscle tone. The behavioral features of a seizure often reflect a function of the cortex where the abnormal electrical activity is found.
Physicians have been able to treat epilepsy by resecting certain brain areas by surgery and by medication. Brain surgery is irreversible, and is ineffective or is associated with neural morbidity in a sizable percentage of cases. Medication is the most prevalent treatment for epilepsy. It is effective in over half of patients, but in the reminder of the patients, the medication is either ineffective in controlling seizures, or the patients suffer from debilitating side effects. A more promising method of treating patients having epileptic seizures is by electrical stimulation of the brain.
Since the early 1970's, electrical brain stimulators have been used which provide more or less constant stimulation, the stimulation largely being unrelated to detected electrical activity.
Electrical stimulation of the nervous system has been used to suppress seizures. A device is described in Cooper et al. for stimulation of the cerebellum. See, “The Effect of Chronic Stimulation of Cerebellar Cortex on Epilepsy and Man,” I. S. Cooper et al in The Cerebellum, Epilepsy and Behavior, Cooper, Riklan and Snyder Edition, Pleman Press, New York 1974. Others have utilized devices which stimulated the centro median nucleus of the thalamus. See, “Electrical Stimulation of the Centro Median Thalamic Nucleous in Control of Seizures: Long Term Studies.” F. Valasco et al, Epilepsia, 36 (1): 63-71, 1995. Chaos Theory has been used to apply stimulation to a seizure focus in vitro to abort the seizure. See, S. Schiff et al, “Controlling Chaos in the Brain,” Nature, Volume 370, Aug. 25, 1994.
As described in U.S. Pat. No. 6,016,449, an improved brain stimulator is implanted in the cranium and has leads terminating with electrodes in contact with brain tissue.
Conventional neurostimulators use fixed rate trains of either monophasic or biphasic electrical pulses of a fixed amplitude to stimulate neural tissue. Neurons in the immediate vicinity of the electrodes are induced to fire (i.e. are recruited) by the electrical pulses thereby modifying the natural electrical activity in the brain. During an epileptic event, there is abnormal synchronization of neural activity in the brain. The present invention improves upon the prior art by varying the timing, amplitude and/or duration of the pulses to more effectively disrupt the synchronized activity. Furthermore, the subject invention analyzes the effect on the brain of the electrical pulses, and decides how to modify the burst parameters in a subsequent burst to most effectively terminate the seizure.
Responsive stimulation, specifically electrical stimulation, that is applied to the brain, has not yet been used to treat patients in long-term studies. This is true even though there are algorithms suitable for detection of the onset of an epileptic seizure. For instance, Qu et al provide an algorithm said to recognize patterns of electrical activity similar to those developed while recording an actual epileptic seizure. See, Qu et al., “A Seizure Warning System for Long-Term Epilepsy Monitoring, Neurology,” 1995; 45:2250-2254. Similarly, Osario, et al. have suggested an algorithm applied to signals from intracranial electrodes with good results. See Osario, et al. “A Method For Accurate Automated Real-Time Seizure Detection,” Epilepsia, Vol. 35, supplement 4, 1995.
As used herein, “epileptiform activity” refers to the manifestation on an EEG (cortical, depth, or scalp) of abnormal brain activity whether associated with clinical manifestations or not. “Stimulation” or “electrical stimulation” means the application of an electric field or electric current to biological tissue.
The inventive device and related process:                1. have improved ability to terminate epileptiform activity.        2. are less likely to initiate epileptiform activity if stimulation is accidentally delivered during a normal EEG.        3. are less likely to generalize ongoing epileptiform activity.        4. are safer since the current density required to affect a larger amount of brain tissue is lower than that found in the prior art.        
None of the cited documents describes the inventive procedures and devices described below.